1. Field of the Invention
The present invention relates to communications systems and, more particularly, to multicarrier systems such as community antenna television systems, or CATV.
1. Background of the Invention
The problems of nonlinear distortions and limited dynamic range in multicarrier, multichannel systems, such as CATV, have received renewed attention with the advent of optical fiber and lasers being used in the distribution of television signals. As is well known, North American CATV systems transmit multiple channels on a set of frequency carriers that may be spaced at 6 MHz intervals. The modulated carriers are combined into a composite multicarrier signal which is distributed to end users from so-called "head-end" transmission stations. Nonlinear distortions generally arise from nonlinearities in the distribution amplifiers between the head-end and the end user causing intermodulation between the various carriers.
In a typical CATV system, losses on the order of 20 decibels (dB) of signal power across a kilometer of coaxial cable is not uncommon. Thus, presently, between the head-end and the television receiver of an end user there may be as many as 20 to 30 amplifiers that recover an attenuated signal transmitted across coaxial cable. Resulting amplifier noise and signal distortions through the system translate into significant losses in dynamic range, i.e., the ratio of the signal to the noise and distortion, of the transmission which substantially degrades with distance.
Yet, while viewers seek more faithful and accurate video and audio reproduction from improvements in noise reduction and dynamic range, demand increases for more television channels. Because optical transmission losses are only of the order of 0.5 dB per kilometer, the industry has been turning to optical signal transmission. A challenge is thus encountered in facilitating reduction of nonlinear distortion components and additional dynamic range with the limitations and cost of optical transducers, for instance, lasers, external optical modulators and light amplifiers.
High speed optical transmitters are increasingly being employed in communications applications, but these applications are generally digital transmissions wherein signal linearity is hardly an issue of concern. However, if digital transmission was used in a CATV system, the costs of installing digital-to-analog converters at each end user would be prohibitive. Thus, due to the fact that essentially no cost is incurred at the end user when the signal format is compatible with existing television receivers, the advantages of carrying analog multichannel waveforms on optical links cannot be overlooked.
Although further improvement in the performance and price of optical transmission devices is needed, recent developments in optical transmitters show significant improvements in their analog characteristics. For instance, improvements have been made in the linearity, optical power and linear dynamic range of semiconductor lasers.
Clearly, digital optical transmission in the Gigabit per second range makes use of the enormous bandwidth available with optical transmitters. On the other hand, the linearity limitations for current practice CATV analog transmissions requiring multicarrier amplitude modulation (AM) place a severe limit on channel capacity and loss budgets, i.e., the allowed loss in signal power that provides adequate dynamic range. At the same time, due to these limitations, analog transmissions do not make full use of the "commodity" of bandwidth available in optical transmission. Hence, it is desirable to find a method of utilizing this high bandwidth capability of optical transmitters for the purposes of distortion suppression and improved dynamic range, while maintaining an analog compatible transmission.
With the objective of reducing intermodulations caused by nonlinear distortions, it is desirable to make a multicarrier system harmonically related and coherent. A harmonically related system is one in which all carriers have frequencies that are integral multiples of a common fundamental frequency. Then, if all carrier frequencies are phase-locked to the appropriate harmonics of the fundamental frequency, the system is said to be coherent. Even the standard CATV frequency plan with frequencies given by f.sub.n =n(6 MHz) +1.25 MHz can be viewed as harmonically related if each carrier is phase-locked to the corresponding harmonics of a 0.25 MHz fundamental frequency. Harmonically related signals, as defined here, need not have consecutive integral multiples of the fundamental frequency. In this regard it should be noted that the terminology in the CATV industry defines Harmonically Related Carrier (HRC) systems as only such systems wherein the frequencies are consecutive harmonics of a 6 MHz fundamental frequency while the case in which the carrier frequencies are phase locked to the standard frequency plan referred to above, are defined as Incrementally Related Carrier (IRC) systems. Here, both types of systems are generally defined as Harmonically Related Carrier systems.
It is well known that in such coherent multicarrier systems, peak-to-peak amplitudes of the composite signal may be reduced by proper phase adjustment of individual carriers without reducing their amplitudes. As amplifier and optical transmitter transfer functions are typically characterized by sigmoidal curves, being nonlinear at low and high input levels and generally linear in the middle region of the curve, a reduction of peak amplitudes allows the multicarrier signal to be transmitted along the linear portion of the transfer function. Therefore, distortions of the composite signal are minimized.
Such coherent multicarrier systems and approaches for adjusting the carrier phases were described in the patent to Switzer, et al., (U.S. Pat. No. 3,898,566) which is incorporated by reference herein. Additional descriptions can be found in two articles by I. Switzer, "Phase Phiddling" published in the 23rd Annual NCTA convention Official Transcript, pp. 2-21, (1974) and "A Harmonically Related Carrier System for Cable Television" published in IEEE Transactions on Communications, Vol. COM-23, pp. 155-166, Jan. (1975) (hereinafter "Switzer (1975)") and in a more recent article by W. Krick, "Improvement of CATV transmission using an optimum coherent carrier system" published in IEEE Transactions on Cable Television, Vol. CATV-5, pp. 65-71, Apr. (1980).
The phase adjustment methods described in the above-cited references were essentially ad hoc and did not yield global optimum phase configurations. The problem with an ad hoc phase tuning method, such as that suggested by Switzer, et al., presents itself in two basic ways. First, the method essentially relies on one variable, the amplitude peak, which is used as a determinant for the direction that each carrier phase must be fine tuned in order to reduce the peak value. The problem that arises here is due to dimensionality: it is difficult to optimize an N degrees of freedom configuration with many local minima by using only one parameter to guide the process.
The second problem is that of stability and interchannel timing drifts due to thermal effects, power variations and component aging. More specifically, even if one is equipped with a list of optimal phases for each channel (for example, a list that may be obtained by a computer numerical simulation of the system described herein), implicit in that representation of phase for different frequencies is the existence of absolute time common to all channel modulators. Thus, the system would still have to be calibrated to ensure that the initial zero crossing of all signals was aligned, and subsequently the phase offsets of the list would have to be adjusted for each carrier. Furthermore, the list of optimal phases may be relevant only for equal amplitude conditions, a situation that can easily be deviated from in actual systems, where relative amplitude drift of up to 2 dB is possible. Furthermore, whenever additional carriers are added to a system, a new "optimal phase list" must be used to recalibrate and readjust the phases, thereby interrupting cable service.
Most important is the fact that while phase-lock modulators accept a phase reference to lock on, their output phase may drift significantly with respect to their input reference phase, thus frustrating any stable phase constellation. These phase drifts, in a CATV transmission system, for example, could result from bandpass filters at the modulator's output stages and the combiner that produces the composite multicarrier signal.
In this regard, it is worth noting that given a minimum peak amplitude, phase optimized system, phase deviations of .+-.5.degree. in all carriers of a simulated 81 channel system was found to cause peak amplitude excursion increases as much as 2 dB above optimum setting. It becomes clear that if one intends to make use of the advantages afforded by phase optimization in coherent systems, a consistent method or system that guarantees optimal operation automatically under variable conditions is now needed. The variable conditions may include thermal drifts, utility power fluctuations, output level drifts or even the failure of one or more of the modulators to produce their assigned carriers.
Consequently, a need presently exists for a multicarrier system and method that increases dynamic range and reduces nonlinear distortions. The system should, in addition, have an analog compatible transmission format which can benefit from the additional bandwidth provided by optical transmission. Furthermore, the system and method should optimize phases automatically under variable conditions.